The lithography machine is the most important central device in integrated circuit manufacturing. Recently immersion lithography becomes the developing trend of lithography technology in integrated circuit manufacturing. The theory of immersion lithography is to fill the area between the wafer and the last glass of the projection objective lens with high refractive index liquid in order to gain larger numerical aperture, and therefore gain higher exposal resolution.
As illustrated in FIG. 1, it discloses the structure of the present lithography machine. In the lithography machine, the main framework 0 supports an illumination system 20, a projection objective lens 10 and a wafer stage 40. The wafer 50 covered with photoresist is positioned on the wafer stage 40. Through the illumination system 20 and the projection objective lens 10, the pattern of integrated circuit is transferred onto the wafer covered with photoresist 50 by means of image exposure, and thereby the exposure process is finished.
The resolution of the exposal pattern is determined by the performance of the lithography machine, while the performance of integrated circuit is determined by the resolution of the exposal pattern. The most important factors which determine the resolution of the exposal pattern are wavelength of the lamp-house, numerical aperture of the projection objective lens and process factor. Increasing numerical aperture can achieve higher resolution under the prerequisite of determined wavelength of the exposal lamp-house. The value of numerical aperture of the projection objective lens is calculated by multiplying the refractive index of image space by the sine value of the angular aperture. FIG. 2 is a schematic view of a sort of immersion exposure technology. As illustrated in FIG. 2, the immersion exposure can increase the resolution through increasing numerical aperture by using a medium of high refractive ratio at the image side. The similar application is popular in the oil immersion microscope, but because of the complexity of the technical conditions, the application of immersion technology in the lithography machine meets many practical difficulties. The first question that must be resolved is to choose the image side medium, for which the optical characteristics in the deep ultraviolet wave band, the interaction with the photoresist, the viscosity, clean ability and damage to the main structure of the lithography machine must be considered. At present ultra-high purity water is the commonest packing medium.
The second question that must be resolved is to maintain the medium in the image field of the lithography machine. Since the area between the projection objective lens 1 and the wafer 5 is an open area, when the lithography machine works, the wafer stage 4 drives the wafer 5 scanning at high speed and carrying out step action, thus the immersion system must provide stable medium condition in the scope of the projection objective lens field of view according to the movement state of the wafer stage 4, and at the same time it must keep the medium waterproof to prevent the liquid from leaking out. Generally, immersion condition is a stable flow field, its velocity and direction of flow is fit for movement of the wafer stage and it also can clean and heat sink.
As to the recently developed dual wafer stages technology, it has two wafer stages working simultaneously in one lithography machine (one wafer stage carries out the scan and exposure steps under the projection objective lens, the other wafer stage carries out wafer loading/unloading, alignment and wafer topography detection. The two wafer stages switch their positions when finishing their corresponding work). Therefore, keeping the immersion lithography machine stable is very important to a dual wafer stage lithography machine.
The work flow of the immersion system can be divided into establishing the immersion environment, removing the immersion environment, and normal exposure. These processes need to cooperate with the whole work flow of the lithography machine. The lithography machine needs to carry out loading or unloading process of the wafer, off axis measuring process and exchanging process of the wafer stages besides the exposure process. All the processes must be carried out under an immersion environment. When the wafer stage is not under the projection objective lens, several problems such as how the status of the immersion system is, whether the flow field needs to be removed must be considered. Removing and establishing the flow field are respectively the processes of emptying the liquid in the immersion environment and filling liquid into the immersion environment from an empty state. These processes cost much time, and the establishment of the flow field on the wafer may greatly concuss the photoresist, which are the cases that the designer wants to avoid. At present, the method used in the semiconductor field, such as the one described in US patent 2005/0036121 A1, changes the maintenance state of the flow field to some extent when the wafer stages are exchanging, adding additional actions and corresponding devices to the wafer stages and the immersion system, complicating the structure of the system.